Prometheus Metrics

Exposes extension metrics in Prometheus text exposition format. No external dependencies. Zero overhead when not enabled - every metric call uses optional chaining on a nullish reference, so V8 short-circuits on a single pointer check.

Setup

// src/lib/server/metrics.js
import { createMetrics } from 'svelte-adapter-uws-extensions/prometheus';

export const metrics = createMetrics({
  prefix: 'myapp_',
  mapTopic: (topic) => topic.startsWith('room:') ? 'room:*' : topic
});

Pass the metrics object to any extension via its options:

import { metrics } from './metrics.js';
import { redis } from './redis.js';
import { createPresence } from 'svelte-adapter-uws-extensions/redis/presence';
import { createPubSubBus } from 'svelte-adapter-uws-extensions/redis/pubsub';
import { createReplay } from 'svelte-adapter-uws-extensions/redis/replay';
import { createRateLimit } from 'svelte-adapter-uws-extensions/redis/ratelimit';
import { createGroup } from 'svelte-adapter-uws-extensions/redis/groups';
import { createCursor } from 'svelte-adapter-uws-extensions/redis/cursor';

export const bus = createPubSubBus(redis, { metrics });
export const presence = createPresence(redis, { metrics, key: 'id' });
export const replay = createReplay(redis, { metrics });
export const limiter = createRateLimit(redis, { points: 10, interval: 1000, metrics });
export const lobby = createGroup(redis, 'lobby', { metrics });
export const cursors = createCursor(redis, { metrics });

Mounting the endpoint

With uWebSockets.js:

app.get('/metrics', metrics.handler);

Or use metrics.serialize() to get the raw text and serve it however you like.

Options

Metric names must match [a-zA-Z_:][a-zA-Z0-9_:]* and label names must match [a-zA-Z_][a-zA-Z0-9_]* (no __ prefix). Invalid names throw at registration time. HELP text containing backslashes or newlines is escaped automatically.

Cardinality control

If your topics are user-generated (e.g. room:abc123), per-topic labels will grow unbounded. Use mapTopic to collapse them:

const metrics = createMetrics({
  mapTopic: (topic) => {
    if (topic.startsWith('room:')) return 'room:*';
    if (topic.startsWith('user:')) return 'user:*';
    return topic;
  }
});

Metrics reference

Pub/sub bus

Presence

Replay buffer (Redis and Postgres)

Rate limiting

Broadcast groups

Cursor

LISTEN/NOTIFY bridge